Why is my Windows Server slow!?
Top 5 Reasons Your Windows Server Might Be Slow!
An application or program is consuming all of your power!
Quick check: Open up Task Manager and look for anything using more CPU, etc than it should.
A service is running that is draining resources
Quick check: These can also be found in Task Manager and can be modified, started, and stopped in the Services app.
A hard disk is failing or has already failed.
Quick check: Most servers will indicate this with a yellow light on the hard drive bay. There are many different hard drive/RAID monitoring programs out there. Depending on your set up is where you would find how to see it.
Malware Infection
Quick check: Run a malware scan!
A necessary update has not been applied
Quick Check: Easy enough just check to see if there are any updates available and apply them as necessary.